Optical lithography has been one of the principal driving forces behind the continual improvements in the size and performance of the integrated circuit (IC) since its inception. Feature resolution down to 0.30 μm is now routine using the 365 nm mercury (Hg) i-line wavelength and optical projection tools operating at numerical apertures above 0.55 with aberration levels below 0.05 λRMS OPD. The industry is at a point where resolution is limited for current optical lithographic technologies. In order to extend capabilities toward sub-0.25 μm, modifications in source wavelength, optics, illumination, masking, and process technology are required and are getting very much attention.
However, as devices get smaller, the photomask pattern becomes finer. Fine patterns diffract light and thus detract from imaging the photomask onto the surface of a wafer. FIG. 1 a shows what happens when a photomask with a fine pattern 6 having a high frequency (pitch 2d is about several microns), is illuminated through a projection lens system 7. The fine pattern 6 is illuminated along a direction perpendicular to the surface thereof and it diffracts the light that passes through the mask 6. Diffraction rays 3-5 caused by the pattern include a zero-th order diffraction ray 5 directed in the same direction as the direction of advancement of the input ray, and higher order diffraction rays such as positive and negative first order diffraction rays 3, 4, for example, directed in directions different from the input ray. Among these diffraction rays, those of particular diffraction orders such as, for example, the zero-th order diffraction ray and positive and negative first order diffraction rays 3, 5, are incident on a pupil 1 of the projection lens system 7. Then, after passing through the pupil 1, these rays are directed to an image plane of the projection lens system, whereby an image of the fine pattern 6 is formed on the image plane. In this type of image formation, the ray components which are contributable to the contrast of the image are higher order diffraction rays. If the frequency of a fine pattern increases, it raises a problem that an optical system does not receive higher order diffraction rays. Therefore, the contrast of the image degrades and, ultimately, the imaging itself becomes unattainable.
As will be shown below, some solutions to this problem rely upon shaping the rays of light impinging the photomask in order to provide off-axis illumination to compensate for the lost contrast due to diffraction. These techniques rely upon optical systems for shaping the rays that illuminate the photomask.
In considering potential strategies for sub-0.25 μm lithography, the identification of purely optical issues is difficult. Historically, the Rayleigh criteria for resolution (R) and depth of focus (DOF) has been utilized to evaluate the performance of a given technology:R=k1λ/NADOF=+/−k2λ/NA2 where k1 and k2 are process dependent factors, λ is wavelength, and NA is numerical aperture. As wavelength is decreased and numerical aperture is increased, resolution capability improves. Considered along with the wavelength-linear and NA-quadratic loss in focal depth, reasonable estimates can be made for system performance. Innovations in lithography systems, materials and processes that are capable of producing improvements in resolution, focal depth, field size, and process performance are those that are considered most practical.
The Hg lamp is a source well suited for photolithography and is relied on almost entirely for production of radiation in the 350-450 nm range. Excimer lasers using argon fluoride (ArF) and krypton fluoride (KrF), which produce radiation at 193 nm and 248 nm, respectively, are also used. DUV lithography at 248 nm is now being implemented into manufacturing operations and may be capable of resolution to 0.18 μm.
The control of the relative size of the illumination system numerical aperture has historically been used to optimize the performance of a lithographic projection tool. Control of this NA with respect to the projection systems objective lens NA allows for modification of spatial coherence at the mask plane, commonly referred to partial coherence. This is accomplished through specification of the condenser lens pupil size with respect to the projection lens pupil in a K ōhler illumination system. Essentially this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illuminator apertures. By controlling the distribution of diffraction information in the objective lens with the illuminator pupil size, maximum image modulation can be obtained. Illumination systems can be further refined by considering variations to fall circular illumination apertures. A system where illumination is obliquely incident on the mask at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination. The resulting two diffraction orders can be sufficient for imaging. The minimum pitch resolution possible for this oblique condition of partially coherent illumination is 0.5 λ/NA, one half that possible for conventional illumination. This is accomplished by limiting illumination to two narrow beams, distributed at selected angles. The illumination angle is chosen uniquely for a given wavelength (λ), numerical aperture (NA), and feature pitch (d) and can be calculated for dense features as sin−1 (0.5 λ/d) for NA=0.5 λ/d. The most significant impact of off axis illumination is realized when considering focal depth. In this case, the zero-th and 1st diffraction orders travel an identical path length regardless of the defocus amount. The consequence is a depth of focus that is effectively infinite.
In practice, limiting illumination to allow for one narrow beam or pair of beams leads to zero intensity. Also, imaging is limited to features oriented along one direction in an x-y plane. To overcome this, an annular or ring aperture has been employed that delivers illumination at angles needed with a finite ring width to allow for some finite intensity. The resulting focal depth is less than that for the ideal case, but improvement over a full circular aperture can be achieved. For most integrated circuit application, features are limited to horizontal and vertical orientation, and a four-zone configuration may be more suitable. Here, zones are at diagonal positions oriented 45 degrees to horizontal and vertical mask features. Each beam is off-axis to all mask features, and minimal image degradation exists. Either the annular or the four-zone off-axis system can be optimized for a specific feature size, which would provide non-optimal illumination for all others. For features other than those that are targeted and optimized for, higher frequency components do not overlap, and additional spatial frequency artifacts are introduced. This can lead to a possible degradation of imaging performance.
When considering dense features (1:1 to 1:3 line to space duty ratio), modulation and focal depth improvement can be realized through proper choice of illumination configuration and angle. For true isolated features, however, discrete diffraction orders would not exist; instead a continuous diffraction pattern is produced. Convolving such a frequency representation with either illumination zones or annular rings would result in diffraction information distributed over a range of angles. Truly isolated line performance is, therefore, not improved with off-axis illumination. When features are not completely isolated but have low density (>1:3 line to space duty ratio), the condition for optimum illumination will not be optimal for more dense features.
Furthermore, the use of off-axis illumination is generally not required for the large pitch values that correspond to low density geometry. As dense and mostly isolated features are considered together in a field, it follows that the impact of off-axis illumination on these features will differ, and a large disparity in dense to isolated feature performance can result.
One approach to generate off-axis illumination is to incorporate a metal aperture plate filter into the fly eye lens assembly of the projection system illuminator providing oblique illumination. A pattern on such a metal plate would have four quadruple openings (zones) with sizing and spacing set to allow diffraction order overlap for specific geometry sizing and duty ratio on the photomask, as disclosed in JP patent Laid-Open (KOKAI) Publication No. 4-267515. Such an approach results in a significant loss in intensity available to the mask, lowering throughput and making the approach less than desirable. Additionally, the four circular openings need to be designed specifically for certain mask geometry and pitch and would not improve the performance of other geometry sizes and spacings. Large levels of mask biasing or mask optical proximity correction (OPC), where mask features are pre-distorted to produce desired image characteristics, would be required to allow for use of this approach with a variety of features. Filtering, by limiting its effective area, reduces the effect of the fly eye diffuser on maximizing illumination uniformity. Illumination uniformity may be degraded. This approach also limits the illumination profile to one having holes in a metal plate. That is, the masking metal must remain contiguous. The previous work in this area describes such methods using either two or four openings in the aperture plate: EP0500393, U.S. Pat. Nos. 5,305,054, 5,673,103, 5,638,211, EP0496891, EP0486316, U.S. Pat. No. 379,252.
Another approach to off-axis illumination using the four-zone configuration, which is disclosed in U.S. Pat. No. 5,627,625, is to divide the illumination field of the projection system into beams that can be shaped to distribute off-axis illumination to the photomask. By incorporating the ability to shape off-axis illumination, throughput and flexibility of the exposure source is maintained. Additionally, this approach allows for illumination that combines off-axis and on-axis (conventional) characteristics. By doing so, the improvement to dense features that are targeted with off-axis illumination is less significant than straight off-axis illumination. The performance of less dense features, however, is more optimal because of the more preferred on-axis illumination for these features. The result is a reduction in the optical proximity effect between dense and isolated features. Optimization is less dependent on feature geometry and more universal illumination conditions can be selected.
A problem with this divided illumination approach is that it requires reconfiguration of the illumination system of a projection tool, a task that is not practical on existing tools or systems designed with other illumination systems. Additionally, the use of divided beam illumination limits the fine control of beam shape, size, and position to that which is possible with optical components utilized in the system. Variations in shape, size, position, number of beams, maximum aperture size, or other feature or lens specific variations to the illumination intensity profile become difficult without significant mechanical modifications. Some variations may not be practical or possible with this approach. This has significantly limited the acceptance or use of this approach in most integrated circuit fabrication operations.